The new Go module system is awesome. It ensures repeatable, deterministic builds of Go code. External module code is cached locally in a read-only directory, which is great for reproducibility. But if you're used to the global mutable namespace that is $GOPATH, there's an obvious question: what if I'm hacking on my program and I want to change one of those external modules?

You might want to put a sneaky log.Printf statement to find out how some internal data structure works, or perhaps try out a bug fix to see if it solves your latest problem. But since all those external modules are in read-only directories, it's hard to change them. And you really don't want to change them anyway, because that will break the integrity checking that the Go tool does when building.

Luckily the modules system provides a way around this: you can add a replace statement to the go.mod file which substitutes the contents of a directory holding a module for the readonly cached copy. You can of course do this manually, but gohack aims to make this process pain-free.

go get github.com/rogpeppe/gohack

or use gobin:

gobin github.com/rogpeppe/gohack

If the module to edit is example.com/foo/bar, run:

gohack get example.com/foo/bar

This will make a copy of the module into $HOME/gohack/example.com/foo/bar and add replace directives to the local go.mod file:

replace example.com/foo/bar /home/rog/gohack/example.com/foo/bar

Note: This copy will not include version control system information so it is best for quick edits that aren't intended to land back into version control.

Run:

gohack get -vcs example.com/foo/bar

This will clone the module's repository to $HOME/gohack/example.com/foo/bar, check out the correct version of the source code there and add the replace directive into the local go.mod file.

Once you are done hacking and wish to revert to the immutable version, you can remove the replace statement with:

gohack undo example.com/foo/bar

or you can remove all gohack replace statements with:

gohack undo

Note that undoing a replace does not remove the external module's directory - that stays around so your changes are not lost. For example, you might wish to turn that bug fix into an upstream PR.

If you run gohack on a module that already has a directory, gohack will try to check out the current version without recreating the repository, but only if the directory is clean - it won't overwrite your changes until you've committed or undone them.